cat4139 Catalyst Semiconductor, cat4139 Datasheet - Page 9

• Manufacturer Part Number: cat4139
• Description: 22v High Current Boost White Led Driver
• Manufacturer: Catalyst Semiconductor

DIMMING CONTROL
There are several methods available to control the
LED brightness.
PWM signal on the SHDN
LED brightness dimming can be done by applying a
PWM signal to the SHDN
repetitively turned on and off, so that the average
current is proportional to the duty cycle. A 100% duty
cycle, with SHDN
LEDs at nominal current. Figure 1 shows a 1kHz
signal with a 50% duty cycle applied to the SHDN
The recommended PWM frequency range is from 100Hz
to 2kHz.
© Catalyst Semiconductor, Inc.
Characteristics subject to change without notice
Figure 1. Switching Waveform
¯¯¯¯¯ always high, corresponds to the
with 1kHz PWM on SHDN
¯¯¯¯¯ input. The LED current is
¯¯¯¯¯ Pin
¯¯¯¯¯
¯¯¯¯¯ pin.
9
Signal
PWM
Filtered PWM Signal
A filtered PWM signal used as a variable DC voltage
can control the LED current. Figure 2 shows the PWM
control circuitry connected to the CAT4139 FB pin.
The PWM signal has a voltage swing of 0V to 2.5V.
The LED current can be dimmed within a range from
0mA to 20mA. The PWM signal frequency can vary
from very low frequency and up to 100kHz.
A PWM signal at 0V DC, or a 0% duty cycle, results
in a max LED current of about 22mA. A PWM signal
with a 93% duty cycle or more, results in an LED
current of 0mA.
Figure 3. Filtered PWM Dimming [0V to 2.5V]
2.5V
Figure 2. Circuit for Filtered PWM Signal
0V
25
20
15
10
5
0
0
10
V
IN
20
0.22µF
3.73kΩ
PWM DUTY CYCLE [%]
R
30
C3
A
40
VIN
SHDN
GND
CAT4139
50
3.1kΩ
R
B
60
FB
SW
V
FB
70
Doc. No. MD-5032 Rev. A
= 300mV
1kΩ
R2
80
CAT4139
90 100
R1
15Ω
LED
Current